Hot air dryers are conventionally used in processing engineering thermoplastic material while it is in pellet form. Most engineering plastics are hygroscopic and tend to adsorb moisture during storage and processing. Nonhygroscopic plastics are susceptible to surface contamination by moisture. In order to enhance the quality of the finished product, the raw plastic should be dried before processing. This is commonly achieved by flowing heated dry air through the plastic material which is held in a drying hopper. In practice, the heated dry air, commonly referred to as "process air", is produced by passing it through a desiccant or molecular sieve for dehumidification and then through a heater before it is delivered to the plastic material. The dry process air is continuously recirculated until the desiccant requires regeneration to remove the adsorbed moisture. As a practical matter, plural desiccant beds in the form of interchangeable cartridges, are used in the drying apparatus. When the desiccant cartridge requires regeneration, it is replaced by a regenerated cartridge and is itself regenerated by flowing heated air therethrough. Following regeneration, it is common practice to cool the desiccant cartridge by flowing cooler air therethrough to reduce the desiccant temperature so that it will more effectively adsorb moisture when it is placed on stream to dehumidify the process air.
In the prior art, drying apparatus is known in which a desiccant bed is automatically moved from the process station to a regeneration station and replaced by a regenerated desiccant bed. This has been accomplished by a continuously rotating desiccant bed with a process air flow path through one sector and a regeneration air flow path through another sector. This arrangement provides for continuous regeneration and avoids the need for complex valves for switching the air flow paths. This is described in advertising bulletin no. 20 dated January 1968 by Thoreson-McCosh, Inc. which describes "Des-i-dri" hopper dryers. A two station indexing hot air dryer is described in the Cooke U.S. Pat. No. 870,546 granted Nov. 12, 1907.
Hot air dryers are also known which utilize three or more cartridges mounted on a turret or carrousel which is indexed automatically to successive stations for process mode, regeneration mode, and cool down mode operation. In this dryer, the cartridges are mounted between two manifolds with rotary valves for gradually changing the flow rate as the carrousel moves the cartridges from one station to the next. This hot air dryer is described in a publication by Con-Air, Inc. as form number 9300CL3-84. An air dryer of this type is also disclosed in the Graff U.S. Pat. No. 3,487,608 granted Jan. 6, 1970.
A general object of this invention is to provide an improved hot air dryer which overcomes certain disadvantages of the prior art.